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CHAPTER 1 INTRODUCTION 1.1 Introduction Modern structures are comparatively tall and slender, have thin walls, are designed for higher stresses and are built at a fast pace. These structures are, therefore, more crack-prone as compared with old structures which used to be low, had thick walls, were lightly stressed and were built at a slow pace. Moreover, moisture from rain can easily reach the inside and spoil the finish of a modern building which has thin walls. Thus measures for control of cracks in buildings have assumed much greater importance on account of the present trends in construction. Cracks in buildings are of common occurrence. A building component develops cracks whenever stress in the component exceeds its strength. Stress in a building component could be caused by externally applied forces, such as dead, live, wind or seismic loads, or foundation settlement or it could be induced internally due to thermal variations, moisture changes, chemical action, etc. Cracks could be broadly classified as structural or non-structural. Structural cracks are those which are due to incorrect design, faulty construction or overloading and these may endanger the safety of a building. Extensive cracking of an RCC beam is an instance of structural cracking. Non-structural cracks are mostly due to internally induced stresses in building materials and these weakening. because

In

course

of penetration

generally

do

of time, however,

not

sometime

directly

result

in structural

non- structural

cracks may,

of moisture through cracks or weathering action, result in

corrosion of reinforcement and thus may render the structure unsafe. Vertical cracks in a long compound wall due to shrinkage or thermal variation is an instance of non-structural cracking. Non-structural cracks, normally do not endanger the safety of a building, but may look unsightly, or may create an impression of faulty work or may give a feeling of instability. Cracks may appreciably vary in width from very thin hair cracks barely visible to naked eye (about 0.01 mm

in width) to gaping cracks 5 mm or more in width. A commonly

known classification1 of cracks, based on their width is: (a) thin— less than 1 mm in width, 1

(b) medium — 1 to 2 mm in width, and (c) wide — more than 2 mm in width. Cracks may be of uniform width throughout or may be narrow at one end, gradually widening at the other. Cracks may be straight, toothed, stepped, map pattern or random and may be vertical, horizontal or diagonal. Cracks may be only at the surface or may extend to more than one layer of materials. Occurrence of closely spaced fine cracks at surface of a material is sometimes called 'crazing'. Internally induced stresses in building components lead to dimensional changes and whenever there is a restraint to movement as is generally the case, cracking occurs. Due to dimensional changes caused by moisture or heat, building components tend to move away from stiff portions of the building which act as fixed points. In case of symmetrical structures, the center of the structure acts as a fixed point and movement takes place away from the center. A building as a whole- can easily move in the vertical direction, but in the horizontal direction,

sub-structure

and foundation exert a restraining action on the

movement of the superstructure. Thus, vertical cracks occur in walls more frequently due to horizontal movement. Volume changes due to chemical action within a component result in either expansion or contraction and as a result cracks occur in the components. Internal stresses in building components could be compressive, tensile or shear. Most of the building materials that are subject to cracking, namely, masonry, concrete, mortar, etc., are weak in tension and shear and thus forces of even small magnitude, when they cause tension or shear in a number, are able to cause cracking. It is possible to distinguish between tensile and shear cracks by closely examining their physical characteristics.

Depending

on certain properties of building materials, shrinkage cracks may be wider but further apart, or may be thin but more closely spaced. As a general rule, thin cracks, even though closely spaced and greater in number, are less damaging to the structure and are not so objectionable from aesthetic and other considerations as a fewer number of wide cracks.

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CHAPTER 2 LITERATURE SURVEY In order to be able to prevent or to minimize occurrence of cracks, it is necessary to understand basic causes of cracking and to have knowledge about certain properties of building materials. Principal causes of occurrence of cracks in buildings are as follows: a. Moisture changes b. Thermal variations c. Elastic deformation d. Creep e. Chemical reaction f. Foundation movement and settlement of soil and g. Vegetation.

2.1 Moisture Changes As a general rule, most of the building materials having pores in their mortar, burnt clay bricks, some stones,

timber,

etc.

Expand on absorbing moisture and

shrink

on

drying. These movements are reversible, that is cyclic in nature and is caused by increase or decrease in the inter-pore pressure with moisture changes, extent of movement depending on molecular structure and 2.1.1

porosity of a material.

Reversible Movement

From consideration of moisture movement of reversible nature, materials could be broadly classified as under: a. Materials having very small moisture movement, as for example, burnt clay bricks, igneous rocks, limestone, marble, gypsum plaster, metals, etc. The use of these materials does not call for many precautions. b. Materials having small to moderate moisture movement, as for example, concrete, sand-lime bricks, sandstones, cement and lime mortars, etc. In the use of these materials some precautions in design and construction are necessary. Based on research, range of reversible moisture movement of some of the commonly used building materials is given in table 1.

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Table 2. 1: Moisture Movement of Some Common Building Materials S.No.

Material

(1)

(2)

Moisture Movement (Dry to saturation percent) (3)

i)

Burnt clay bricks, limestone

0.002 to 0.01

ii)

Hollow clay bricks, terra cota

0.006 to 0.016

iii)

iv) v)

Expanded clay concrete,

0.017 to 0.04

cinder concrete Sandstone, sand-lime

0.01 to 0.05

bricks, concrete block's Foam cellular concrete

vi)

vii)

0.04 to 0.05

Cast-stone, dense concrete, cement lime mortars Auto-clayed aerated

0.02 to 0.06

0.03 to 0.08

concrete, clinker concrete

viii)

Marble

Negligible

ix)

Wood along grain

0.000 8

Wood across grain —

x)

5 to 15

tangential

xi)

Wood across grain —radial

3 to 5

Initial drying shrinkage in cement and lime a product which is partly irreversible is 50 percent more than the values of reversible shrinkage given above. Data for items (i) to (vii) are reproduced from 'principles of modern buildings'. Volume i and for items (viii) to (xi) from 'common defects in buildings 11. 2.1.2

Initial Shrinkage

Initial shrinkage, which is partly irreversible, normally occurs in all building materials or components that are cement/lime- based, for example, concrete, mortar, masonry units, masonry and plasters. This shrinkage is one of the main causes of cracking in structures. Influence of these factors on shrinkage is as follows: a. Cement content —as a general rule, richer the mix, greater the drying shrinkage. Conversely, larger the volume of aggregate in concrete, lesser the shrinkage. For the 4

range of aggregate content generally used for structural concretes, increasing the volume of aggregates by 10 percent can be expected to reduce shrinkage by about 50 percent. Relation between mix proportion and shrinkage is depicted in fig.2.1.

Fig 2. 1: Relation between Mix Proportion and Drying Shrinkage of Cement Concrete Mortar (Reproduced From ‘Principles of Modern Buildings Volume 17) b. Water content — greater the quantity of water used in the mix, greater the shrinkage. Thus a wet mix has more shrinkage than a dry mix which is otherwise similar. That explains why a vibrated concrete, which has low slump, has lesser shrinkage than a manually compacted concrete, which needs to have greater slump. In terrazo and concrete floors, use of excess water in the mix (commonly resorted to by masons to save time and lab our on compaction and screeding) is one of the principal causes of cracking in such floors. A typical relation between water content and drying shrinkage is shown in fig. 2.2.3

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Fig 2. 2: Effect of Variation in Water Content of Concrete on Drying Shrinkage (Based On Graph Given In ‘Control of Cracking in Concrete Structures) c. Aggregates — by using the largest possible maximum size of aggregate in concrete and ensuring good grading, requirement of water for concrete of desired workability is reduced and the concrete thus obtained has less shrinkage because of reduction in the porosity of hardened concrete. Any water in concrete mix in excess of that required for hydration of cement, to give the desired workability to the mix, results in formation of pores when it dries out, thus causing shrinkage. Fig 2.3 illustrates the effect of aggregate size on water requirement. For the same cement-aggregate ratio, shrinkage of sand mortars is 2 to 3 times that of concrete using 20 mm maximum size aggregate and 3 to 4 times that of concrete using 40 mm maximum size aggregate.

Fig 2. 3: Effect of Aggregate Size on Water Requirement of Concrete d. Use

of accelerators — use

of calcium chloride

as

accelerator

in

concrete

appreciably shrinkage increases—being up to 50 percent with 0.5 to 2.0 percent addition of calcium chloride. Shrinkage could be much more if proportion of calcium chloride is higher. Moreover, it has some corrosive effect on reinforcement in concrete. 6

e. Curing—curing also plays an important part in limiting shrinkage. If proper curing is started as soon as initial set has taken place and it is continued for at least 7 to 10 days, drying shrinkage is comparatively less, because when hardening of concrete takes place under moist environments, there is initially some expansion which offsets a part of subsequent shrinkage. Steam curing of concrete blocks at the time of manufacture reduces their liability to shrinkage as high temperature results in precarbonation. f. Presence of excessive fines—presence of excessive

fines—silt,

clay,

dust —in

aggregates has considerable effect on extent of shrinkage in concrete. Presence of fines increases specific surface area of aggregates and consequently the water requirement. Rightly, therefore, specifications for fine and coarse aggregates for concrete lay much emphasis on cleanliness of aggregates and stipulate a limit for the maximum percentage of fines in aggregates which is 3 percent for coarse as well as uncrushed fine aggregate according to is: 383-1970 7 . g. Humidity — extent

of shrinkage also, depends on relative humidity of ambient

air. Thus, shrinkage is much less in coastal areas where relative humidity remains high throughout the year. Low relative humidity may also cause plastic shrinkage in concrete. h. Composition of cement — chemical composition of cement used for concrete and mortar also has some effect on shrinkage. It is less for cements having greater proportion of tricalcium silicate and lower proportion of alkalis like sodium and potassium oxides. Rapid hardening cement has greater shrinkage than ordinary Portland cement. i. Temperature — an important factor which influences the water requirement of concrete and thus its shrinkage is the temperature of fresh concrete. This is illustrated in fig.2.4 based on studies made by bureau of reclamation, USA.

7

Fig 2. 4: Effect of Temperature of Fresh Concrete on Water Requirement If temperature of concrete gets lowered from 38°c to 10°c it would result in reduction of water requirement to the extent of about 25 liters per cubic meter of concrete for the same slump. It, thus, follows that in a tropical country like India, concrete work done in mild winter. 2.1.3

Cracks In Freshly Laid Cement Concrete

In freshly laid cement concrete pavements and slabs, sometimes cracks occur before concrete has set due to plastic shrinkage. This happens if concrete surface loses water faster than bleeding action brings it to top of concrete at the surface results in shrinkage and as concrete in plastic state cannot resist any tension; short cracks develop in the material. These cracks may be 5 to 10 cm in depth and their width could be as much as 3 mm. Once formed these cracks

stay

and may, apart from being unsightly affect

serviceability of the job. In order, to prevent plastic shrinkage of concrete, it is necessary to take steps so as to slow down the rate of evaporation from the surface of freshly laid concrete. Immediately after placing of concrete, solid particles of the ingredients of concrete begin to settle down by gravity action and water rises to the surface. This process — known as bleeding—produces a layer of water at the surface and continues till concrete has set. As long as rate of evaporation is lower than the rate of bleeding, there is a continuous layer of water at the surface, as evidenced by the appearance of ‘water sheen' on the surface and shrinkage does not occur.

8

2.1.4

Cracks In Brick Work Due To Expansion

When clay bricks (or other clay products) are fired, because of high temperature (900°c to 1000°c), not only intermolecular water but also water that forms a part of the molecular structure of clay, is driven out. After burning, as the temperature of bricks falls down, the moisture- hungry bricks start absorbing moisture from the environment and undergo gradual expansion, bulk of this expansion being irreversible. Extent of irreversible expansion depends on the nature of soil, that is, its chemical and mineralogical composition and the maximum temperature of burning. When bricks are fired at very high temperature, as in the case of engineering bricks, because of fusion of soil particles, there is discontinuity in the pores and as a result, water absorption and moisture movements are less. 2.1.5 i.

Measures For Controlling Cracks Due To Shrinkage To avoid cracks in brickwork on account of initial expansion, a minimum period varying from 1 week to 2 weeks is recommended by authorities for storage of bricks after these are removed from kilns.

ii.

Shrinkage cracks in masonry could be minimized by avoiding use of rich cement mortar in masonry and by delaying plaster work till masonry has dried after proper curing and has undergone most of its initial shrinkage.

iii.

Use of precast tiles in case of terrazo flooring is an example of this measure. In case of in-situ/terrazo flooring, cracks are controlled by laying the floor in small alternate panels or by introducing strips of glass, aluminum or some plastic material at close intervals in a grid pattern, so as to render the shrinkage cracks imperceptibly small.

iv.

In case of structural concrete, shrinkage cracks are controlled by use of reinforcement, commonly

termed

as

'temperature reinforcement'.

This

reinforcement is intended to control shrinkage as well as temperature effect in concrete and is more effective if bars are small in diameter and are thus closely spaced, so that, only thin cracks which are less perceptible, occur 6 . v.

To minimize shrinkage cracks in rendering/plastering, mortar for plaster should not be richer than what is necessary from consideration of resistance to abrasion and durability 9

2.2 Thermal Variations It is a well-known phenomenon of science that all materials, more or less, expand on heating and contract on cooling. Magnitude of movement, however, varies for different materials depending on their molecular structure and other properties. When there is some restraint to movement of a component of a structure, internal stresses are set up in the component, resulting in cracks due to tensile or shear stresses. Extent of thermal movement in a component depends on a number of factors, such as temperature variation, dimensions, co-efficient of expansion and some other physical properties of the materials. 

Thermal co-efficient for brickwork as given above is for movement in horizontal direction; for movement of brickwork in the vertical direction, coefficient is 50 percent higher.



Data contained in this table is from ‘principles of modern buildings'. Vol. I1 excepting item (iii), which is from

the 'performance of high" rise masonry

structures 18 and item (vi) which is from ' thermal movements and expansion joints in buildings 17 . Coefficients of thermal expansion of some of the common building materials are given in table 3. Table 2. 2: Coefficient of Thermal Expansion of Some Common Building Materials (Within the Range 0°C to I00°C) S. No

MATERIAL

(1)

(2)

Co-EFFICENT OF THEMAL EXPANSION (3)

i)

Bricks and brickwork

5 to 7

ii)

Cement mortar and

10 to 14

Concrete iii)

Sand-lime bricks

iv)

Stones:

11 to 14

a) Igneous rocks

8 to10

(Granite, etc.) b) Limestone’s

2.4 to 9

10

v)

vi) 2.2.1

c) Marble

1.4 to 1 1

d) Sandstones

7 to 16

e) Slates

6 to 10

Metals: a) Aluminum

25

b) Bronze

17.6

c) Copper

17.3

d) Lead

29

e) Steel and iron

11 to 13

Wood Factors Affect The Thermal Movement

Other factors which influence the thermal movement of component are: color and surface characteristics, thermal conductivity, provision of an insulating or protective layer and internally generated heat, as discussed below: 2.2.1.1

Color And Surface Characteristics

Dark colored and rough textured materials have lower reflectivity than light colored and smooth textured materials and thus, for the same exposure conditions, gain of heat and consequently rise in temperature of the former is more. 2.2.1.2

Thermal Conductivity

Low thermal conductivity of a component, which is subject to solar radiation, produces a thermal gradient in the component, resulting in warping of the component. In case of concrete roof slabs, as the material has low conductivity, thermal gradient is quite appreciable and that causes the slab to arch up and also to move outward due to heat from the sun. This results in cracks in external walls which support the slab and in the internal walls that are built up to the soffit of the slab. It is thus very necessary to provide a layer of adequate thickness of a suitable material preferably with a good reflective surface over concrete roof slab in order to minimize cracking in walls. 2.2.1.3

Provision Of An Insulating Or Protective Layer

If there is a layer of an insulating or heat absorbing material acting as protective cover to a, component, shielding it from sun rays, heat gain or loss of the component is considerably reduced and thus its thermal movement is lessened. 11

2.2.1.4

Internally Generated Heat

Rise of temperature in fresh concrete can take place not only due to heat gained from an external source but also due to heat generated within the material by hydration of cement. Reflectivity co-efficient of some of the commonly used building materials

are

given in table 4. Table 2. 3: Heat Reflectivity Co-Efficient Of Some Common Building Materials S .No

Material

Reflectivity Co-efficient

(1)

(2)

(3)

i)

Asphalt

0.09 To. 17

ii)

G.I. sheets

0.10 to 0.36

iii)

Asbestos cement sheets

0.29 to 0.58

iv)

Brickwork (exposed)

0.30 to 0.58

v)

Cement mortar and concrete

0.34 to 0.65

vi)

Granite (reddish)

0.45

vii)

Aluminum paint

0.46

viii)

Aluminum sheets

0.47

ix)

Marble (white)

0.56

x)

White paint

0.71

xi)

Whitewash

0.79 to 0.91

2.2.2

Measures For Controlling Cracks Due To Shrinkage

Some general measures for prevention of cracks due to thermal movement are given below: a. Wherever feasible, provision should be made in the design and construction of structures for unrestrained movement of parts, by introducing movement joints of various types, namely, expansion joints, control joints and slip joints. b. Even when joints for movement are provided in various parts of a structure, some amount of restraint to movement due to bond, friction and shear is unavoidable. Concrete, being strong in compression, can stand expansion but, being weak in tension, it tends to develop cracks due to contraction and shrinkage, unless it is provided with adequate reinforcement for this purpose. . Members in question could thus develop cracks on account of contraction and shrinkage in the latter direction.

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It is, therefore, necessary to provide some reinforcement called 'temperature reinforcement" in that direction. c. Over flat roof slabs, a layer of some insulating material or some other material having good heat insulation capacity, preferably along with a high reflectivity finish, should be provided so as to reduce heat load on the roof slab. d. In case of massive concrete structures, rise in temperature due to heat of hydration of cement should be controlled. 2.2.3

Provision Of Joints In Structure

Movement joints in structures are introduced so that unduly high stresses are not set up in any part of a structure, and it may not develop unsightly cracks. When a joint permits expansion as well as contraction it is termed as 'expansion joint 5 , when it allows only contraction, it is termed as 'control joint' and when the joint permits sliding movement of one component over another it is termed as 'slip joint’. Information given in table 5 is intended to serve as a general guide in this regard. Table 2. 4: A General Guide for Provision of Movement Joints in Buildings Type of Structure 1.

2.

3.

4.

5.

Movement of Joints Provide 20 to 25 mm wide, joint at 10

RCC roof slab

to 20 M apart

Supports for RCC slabs 4 to 6m

Provide slip joint between slab and

length

bearing wall.

RCC framed and bearing structure Junction between old and new

Provide 25 to40mm wide expansion joints at 30 to 45 M interval Provide vertical slip joints

structure

Expansion joint 5 to 8 mm at 5 to 8 M

Compound walls

interval and change of direction Provide 20 to 25 mm wide joints at 25 m to 40 m interval with control joints

6.

Concrete pavement

at 5 to 8 m. In cross direction control joints have to be provided at 3 to 5 m intervals. 13

7.

Chajja

8.

RCC Railing

Provide expansion joint 5 to 8 mm wide at 4 to 6 M interval. Provide expansion joints 5 to 8 mm wide at 6 to 9 m interval.

Note — for seismic zones iii, iv & v, expansion joints have to be much wider for which is: 4326-1976 ‘code of practice for earthquake resistant design and construction of buildings (first revision) should be referred.

2.3 Elastic Deformation Structural components of a building such as walls, columns, beams and slabs, generally consisting of materials like masonry, concrete, steel, etc., undergo elastic deformation due to load in accordance with hook's law, the amount of deformation depending upon elastic modulus of the material, magnitude of loading and dimensions of the components. This deformation, under circumstances such as those mentioned below, causes cracking in some portions: a. When walls are unevenly loaded with wide variations in stress in different parts, excessive shear strain is developed which causes cracking in walls. b. When a beam or slab of large span undergoes excessive deflection and there is not much vertical load above the supports, ends of beam/slab curl up causing cracks in supporting masonry.

Fig 2. 5: Details of Bearing at the Supports for a Roof Slab of Large Span 14

c. When two materials, having widely different elastic properties, are built side by side, under the effect of load, shear stress is set up at the interface of the two materials, resulting in- cracks at the junction. Sahlin has recommended use of cellular plastic pad with a layer of tar-felt under the slab bearing together with a filling of mineral wool between the slab and brick cover in the upper-most one or two stories of a multistoried building having large spans so as to avoid cracks at supports due to deflection, and shrinkage of slab as shown in Fig 2.21.

2.4 Movement Due To Creep Some building items, such as concrete, brickwork and timber, when subjected to sustained loads not only undergo instantaneous elastic deformation, but also exhibit a gradual and slow time-dependent deformation known as creep or plastic strain. The latter is made up of delayed elastic strain which recovers when load is removed, and viscous strain which appears as permanent set and remains after removal of load. This phenomenon known as creep is explained in fig. 2.18.

Fig 2. 6: Phenomenon of Creep for a Visco-Elastic Material 15

2.4.1 In

Beneficial Effect Of Creep certain

situations,

creep

has

a beneficial effect on the performance of

materials, as it tends to relieve shrinkage and thermal stresses. For example, seasonal variations in temperature being gradual and slow, have less damaging effect on a structure because of creep in the material. Similarly, if process of curing of concrete and masonry is discontinued

gradually,

thereby

slowing down the pace of drying of these items,

shrinkage stress gets relieved due to creep, and cracking due to shrinkage is lessened. 2.4.2

Measures For Prevention Of Cracks Due To Creep

Though it may not be possible to eliminate cracking altogether, following measures will considerably help in minimization of cracks due to elastic strain, creep and shrinkage: 1. Use concrete which has low shrinkage and low slump. 2. Do not adopt a very fast pace of construction. 3. Do not provide brickwork over a flexural RCC member (beam or slab) before removal of centering, and allow a time interval of at least 2 weeks between removal of centering and construction of partition or panel wall over it. 4. When brick masonry is to be laid abutting an RCC column, defer brickwork as much as possible. 5. When RCC and brickwork occur in combination and are to be plastered over, allow sufficient time (at least one month) to

RCC and- brickwork to undergo initial

shrinkage and creep before taking up plaster work. Also, either provide a groove in the plaster at the junction or fix a 10 cm wide strip of metal mesh or lathing over the junction to act as reinforcement for the plaster. 6. In case of RCC members which are liable to deflect appreciably under load, for example, cantilevered beams and slabs, removal of centering and imposition of load should be deferred as much as possible (at least one month) so that concrete attains-sufficient strength, before it bears the load.

2.5 Movement Due To Chemical Reaction Certain chemical reactions in building

materials

result

in

appreciable increase in

volume of materials, and internal stresses are set up which may result in outward thrust and formation of cracks. The materials involved in reaction also get- weakened in strength. Commonly occurring instances of this

phenomenon are: sulphate attack on cement 16

products, carbonation

in

cement-based materials, and corrosion of reinforcement in

concrete and brickwork, and alkali-aggregate reaction.

2.5.1 Effect Of Chemical Reaction 2.5.1.1

Due To Sulphate Attack

Soluble sulphate which are sometimes present in soil, ground water or clay bricks react with tricalcium aluminate content of cement and hydraulic lime in the presence of moisture and form products which occupy much bigger volume than that of the original constituents. This expansive reaction results in weakening of masonry, concrete and plaster and formation of cracks. For such a reaction to take place, it is necessary that soluble sulphates, tricalcium aluminate and moisture — all the three are present. Severity of sulphate attack in any situation depends upon: a) Amount of soluble sulphates present b) Permeability of concrete and mortar c) Proportion of tri-calcium aluminate present in the cement used in concrete and mortar and Sulphate attack on concrete and mortar of masonry in foundation and plinth would result in weakening of these components and May, in course of time, result in unequal settlement of foundation and cracks in the superstructure. If brick aggregate used in base concrete of flooring contains too much of soluble sulphates (more than 1 percent) and water table is high so as to cause long spells of dampness in the base concrete, the latter will in course of time swell up resulting in upheaving and cracking of the concrete floor 15.

17

Fig 2. 7: Cracking and Upheaving Of a Tile Floor Due To Sulphate Action in Base Concrete Upheaving of a concrete tile floor due to sulphate attack is shown in Fig 2.7.  General Measures For Avoidance Of Sulphate Attack: a. In

case

of

structural

concrete

in foundation, if sulphate content in soil

exceeds 0.2 percent or in ground water exceeds 300 ppm, use very dense concrete and either increase richness of mix to 1:1/5:3 or use sulphate resisting Portland cement/super-sulphated cement or adopt a combination of the two methods depending upon the sulphate content of the soil. b. For superstructure masonry, avoid use of bricks containing too much of soluble sulphates (more than 1 percent in exposed situations, such as parapets, free standing walls and masonry in contact with damp soil as in foundation and retaining walls; and more than 3 percent in case of walls in less exposed locations) and if use of such bricks cannot be avoided, use rich cement mortar (1:1/2:4.5 or 1:1/4 :3) for

18

masonry as well as plaster or use special cements mentioned earlier and take all possible precautions to prevent dampness in masonry. 2.5.1.2 Under

Due To Corrosion Of Reinforcement most

conditions

concrete provides good protection to steel embedded in it.

Protective value of concrete depends upon high alkalinity and relatively high electrical resistivity of concrete, extent of protection, depending upon the quality of concrete, depth of concrete cover and workmanship. As steel gets corroded, it increases in volume thus setting up internal stress in concrete. In course of time it first causes cracks in line with the direction of reinforcement and later causes spalling of concrete, dislodging cover of reinforcement from the body of the concrete, thus seriously damaging the structure. To prevent such cracking and premature deterioration, it is desirable to specify concrete of richer mix (say 1:1/5:3) for thin sections in exposed locations and to take special care about grading, slump, compaction and curing of concrete 6 .

Fig 2. 8: Cracking Due To Corrosion of Reinforcement

2.6 Foundation Movement And Settlement Of Soil Shear cracks in buildings occur when there is large differential settlement of foundation either due to unequal bearing pressure under different parts of the structure or due to bearing pressure on soil being in excess of safe bearing strength of the soil or due to low factor of safety in the design of foundation. 2.6.1

Effect Of Expansive Soil On Building

Buildings constructed on shrinkable clays (also sometimes called expansive soils) which swell on absorbing moisture and shrink or drying as a result of change in moisture 19

content of the soil, are extremely crack prone and special measures are necessary to prevent cracks in such cases. Effect of moisture variation generally extends up to about 3.5 m depth from the surface and below that depth it becomes negligible. Roots of fast growing trees, however, because drying and shrinkage of soil to greater depth. Effect of soil movement can be avoided or considerably reduced by taking the foundation 3.5 m deep and using moorum, granular soil or quarry spoil for filling in foundation trenches and in plinth. Variation in moisture

content of soil under the foundation

of a

building

could be considerably reduced by providing a waterproof apron all-round the building. Use

of

under-reamed

piles

in foundation for construction on shrinkable soils has

proved effective and economical for avoiding cracks and other foundation problems. It is necessary that bulb of the pile is taken to a depth which is not much affected by moisture variations. 2.6.2

Provision Of Horizontal Extension With An Expansion Joint

Sometimes it becomes necessary to make a

horizontal

extension

to

an

existing

structure. Since foundation of a building generally undergoes some settlement as load comes on the foundation, it is necessary to ensure that new construction is not bonded with the old construction and the two parts (old and new) are separated by a slip or expansion joints right from bottom to the top, as otherwise when the newly constructed portion undergoes settlement, an unsightly crack may occur at the junction. Care should also be taken that in the vicinity of the old building; no excavation below the foundation level of that building is made. When plastering the new work a deep groove should be formed separating the new work from the old. If the existing structure is quite long (20 to 25 m), the old and new work should be separated by an expansion joint with a gap of about 25 to 40 mm so as to allow some room for unhindered expansion of the two portions of the building.

2.7 Cracking Due To Vegetation Existence of vegetation, such as fast growing trees in the vicinity of compound walls can sometimes cause cracks in walls due to expansive action of roots growing under the foundation. Roots of a tree generally spread horizontally on all sides to the extent of height of the tree above the ground and when trees are located close to a wall; these should always be viewed with suspicion. 20

Fig 2. 9: Cracking Of a Compound Wall Due To Growing Roots under the Foundation

2.8

General methods for repairing of cracks

Various methods are used for repairing of cracks some of which are discussed below: 2.8.1

Routing and sealing:

This is the simplest and most common technique used for repairing of cracks. In this method the cracks are made widened / enlarge along its exposed face then felling and sealing it with a suitable joint sealant. It does not required skill labor. It is most applicable for plate surfaces i.e. floor and pavements. The surface should be clear with air jet and dried before placing the sealant. The sealant may be epoxies, silicon, asphaltic material or a polymer motor. 2.8.2

Stitching

This process drilling holes on both sides of the cracks, the hole are then cleaned, then Ushape metal, staples shaped metal of suitable length or anchored into the holes, which is filled with non-shrinkable grout or epoxy resin to get clamped well. The stitching will prevent crack from further propagation. 2.8.3

Additional Reinforcement

This method is also called conventional reinforcement. This method is mostly use for repairing or girders of bridge. This technique consist of drilling the holes such that they intersect the cracks plane at approximately 90º, then filling the hole in crack with injected epoxy and placing reinforcing for into the drilled hole. 2.8.4

Grouting

In this method Portland cement grout is used. In dams in thick concrete walls may be repaired by filling with Portland cement grout. This method is effective and stopping water leakage but not strictly bound the cracked sections.

21

(Grout may consist of 1 part of cement and 5 parts of water or 1 part of cement and 1 part of water) 2.8.5

Gravity Filling:

Low viscosity monomers resins can be used to repair cracks with surface wider about 0.3 to 2 mm by gravity filling. Hi molecular weight Meta crystal, urethanes and some low viscosity epoxy have been used successfully the lower the viscosity the finer the crack can be filled. 2.8.6

Dry Packing

It is the hand placement of low water motor into the slot or cracks. The motor should be placed in the layer about 3/8 inches thick. Each layer should be compact with the help of hammer or blunt stick. Curing is done for 14 days (ratio 1 cement 5/2 sand and water only to stuck the binding material together) 2.8.7

Epoxy Injection

In this solution all cracks should be injected with liquid epoxy resin type araldite of grade (GY 25) or equivalent. This solution is acceptable if the factor of safety is reduced from 3.0 to 1.5 and no additional loads to the roof in the future. Also, the beams must be tested after the completion of the repair work by using load test.

22

CHAPTER 3 CAUSES The majority of low-rise buildings in the UK are constructed using brick, concrete block or stone with mortar joints. These materials possess significant compressive strength but their ability to accommodate tension is limited. As a consequence if tension stress develops cracking frequently occurs. There are numerous possible causes of cracking. There may be a single cause or a combination of several causes, or one primary cause with several contributory factors. It is beyond the scope of this paper to list more than just a few. Some common causes of cracking are listed below:

3.1 Foundation Subsidence: Foundation subsidence is the downward (or upward or lateral) movement of the foundation and takes place independent of the load from the building foundation. Typical causes of subsidence in the UK are: 

The expansion and shrinkage of clay soils with changes in moisture content.



The collapse of former mine workings.



Leaking drains which causes “washout” or softening of the soils supporting the foundation.



Landslip of sloping ground.



Made or filled ground.



Peat.

3.2 Foundation Settlement: Foundation settlement is the downward movement of the foundation caused by the imposed load from the building. This takes place on loose, soft and highly compressible soils where the load imposed from the foundation overstress the soils supporting the foundation. Although subsidence or settlement can cause cracking and distortion to the fabric of the building, it is differential settlement that causes the more serious damage. 3.2.1

Incompatibility Of Building Materials:

The use different materials in a building can result in cracks occurring. For example, supporting brittle concrete block walls on timber beams or lintels. The timber is flexible

23

and will shrink with reduction of moisture content as well as experiencing long-term load creep deflection. This can result in cracking of the concrete block wall. 3.2.2

Chemical Reaction Of Materials:

Many materials used in the construction of a building are susceptible to chemical reactions. For example the cracking that occurs in the horizontal bed joint in brick walls due to sulphate from the bricks reacting with the mortar. 3.2.3

Thermal Movements.

Tensile and compressive stresses develop within the building elements due to temperature changes. The magnitude of the stress depends on the coefficient of thermal expansion of the material. Cracks can occur if the building element is restrained and lacks sufficient joints to accommodate the movement. 3.2.4

Changes In Moisture Content:

There is a significant change in the moisture content of many building materials after construction especially during the first few months. Clay bricks initially expand and concrete blocks experience shrinkage following curing. If the materials are used together monolithically, stresses can develop with resulting cracks. 3.2.5

Structural Instability:

The structural failure of the building can cause cracks as well as exerting stresses on individual elements causing further cracking. There are many published documents describing in more detail the various causes of cracking in low-rise buildings (ISTRUCTE, 2000), (BONSHOR & BONSHOR, 1996), (BRE, 1991). The objective of the initial inspection, the crack survey and monitoring and gathering data is to enable you to collect sufficient evidence to support an objective opinion on the significance of the cracking.

3.3 Deciding If The Cracking Is Significant The client has asked the question – “is that crack serious?” In the midst of the collecting evidence it is an easy matter to lose sight of the original concern of the client. The results of the initial inspection, the crack survey, crack monitoring and gathering data should answer the following questions: 24

3.3.1

Is The Movement Across The Crack Static?

This can point to the following possible causes: 

The initial “bedding in” of foundations of a new building.



Initial shrinkage of construction materials.



Load induced deflection of beams and slabs as a result of imposed dead load.

3.3.2

Is The Movement Across The Crack Cyclic?

This can point to the following possible causes: 

Thermal movement.



Seasonal clay shrinkage and swelling affecting shallow foundations.



The formation of ice lenses in certain soils causing the effects of expansion and shrinkage on shallow foundations.

3.3.3

Is The Movement Across The Crack Progressive?

This can point to the following possible causes: 

Roof spread of a pitched roof.



foundation subsidence and / or settlement due to



leaking drains



filled ground



peat and compressible soils



clay shrinkage and swelling caused by trees



Hillside creep and instability.



chemical reaction



Sulphate attack.



carbonation



alkali silica reaction



wall tie corrosion

The cracks can be classified into three categories: 3.3.4

Is The Crack Only Aesthetic?

Some cracks only affect the aesthetic appearance of the building and do not affect the functioning or the building nor do the cracks cause structural instability.

25

Fig 3. 1: This Crack Is Aesthetic Damage Only 3.3.5

Is The Crack Affecting The Serviceability?

If the cracking affects the functioning of the building or individual elements the damage is described as serviceability damage. For example the building is no longer watertight, the functioning of the drains and the services are impeded, the glazing in the windows break or the doors do not open or close.

26

Fig 3. 2: The Crack Width Adjacent to the Window Frame Is About 60mm. Clearly The Building No Longer Watertight And The Thermal Insulation Is Being Compromised. In Time The Construction Materials Will Degrade. 3.3.6

Is The Cracking Affecting The Stability?

It is rare for a building or structure to suffer sufficient damage for it to affect the overall stability, but if movement is allowed to continue unchecked, individual elements may become unstable for example the reduced bearing of a beam due to differential movement at its support.

27

Fig 3. 3: The Panel Of Wall On The Left Of The Crack Is Leaning Outwards. There Is Only Minimal Lateral Restraint At The Gable And First Floor. For example, if the cracking is found to be only aesthetic and static, the remedial work is usually simple and inexpensive and there is no need for further monitoring. However remedial work becomes potentially more complex and expensive if the cracking is found as a result of the monitoring to aesthetic and progressive, because the movement may progress from only aesthetic damage to affecting serviceability and ultimately the stability of the building. This methodology will not eliminate the need through analysis and investigation to identify the cause or causes of cracking and specify appropriate remedial work. However if it is applied it will result in a more rational and consistent approach to the assessment of cracking in buildings. Its application will result in recommendations that reflect the severity of the cracking, the need for urgent remedial work or whether further monitoring is required.

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CHAPTER 4 REPAIRING Having completed the investigation work and drawn the conclusion concerning the required remedy to the problem, next step was to develop a proper repair system. In order to solve the problem, the two alternatives were proposed:

4.1 Epoxy Injection In this solution all cracks should be injected with liquid epoxy resin type araldite of grade (GY 25) or equivalent. This solution is acceptable if the factor of safety is reduced from 3.0 to 1.5 and no additional loads to the roof in the future. Also, the beams must be tested after the completion of the repair work by using load test.

4.2 Epoxy Injection Easing The Section Of The Beams This solution for keeping the factor of safety equals 3.0 as it is required by buildings code requirements for reinforced concrete. In this solution a new layer of reinforcement should be fitted around the beams and a layer of concrete pumped using shot Crete. As an alternative for this solution, steel plates can be installed and fixed with beams by using epoxy resin bonded to increase the strength of the beams for the flexure and shear strength. After reviewing the proposed solutions and the conditions of the Building, the solution of epoxy injection was selected.

4.3 Repair Work As mention before epoxy injection solution was selected. The following steps were followed to carry out the repair work: a) The cracks were cleaned thoroughly with compressed air. b) Entry ports (nipples) were installed using adhesive material, Spacing 40 cm between the two nipples. For some cracks which continue to the other side of the beam, the nipples were installed in both sides with staggered distribution. c) The cracks surfaces were sealed with epoxy in order to keep the Injected epoxy from leaking out.

29

d) After drying the sealed epoxy, injection process was started using Epoxy. The injection was executed using injected system for epoxy. The injection was started at the lowest nipple until the epoxy level reaches the nipple above. The lower nipple was then capped, and the process was repeated at the higher nipples until the crack completely filled and all nipples were capped (Fig 3). The injection process was continued until all cracks completely injected. e) After drying the epoxy, the nipples and surfaces sealed were removed. f) Information about each crack was recorded in tables include the Crack

length,

crack width, quantity of epoxy injected, and also drawings showed the location of the cracks on the beam. (Sample of the table is attached in the appendix.)

4.4 Repair Work Evaluation According to building code requirements for reinforced concrete, a strength evaluation may be required if the materials are considered to be deficient in quality, if there is evidence indicating faulty construction, if a structure has deteriorated, if a building will be used for a new function, or if, for any reason, a structure or a portion of it does not Appear to satisfy the requirements of the code.

Fig 4. 1: Nipples & Cracks after Injection After the completion of epoxy injection work, load test was carried out on the repaired beams in order to ensure the effectiveness of the repair work and to ensure the integrity for those beams.

30

The principal aim of load test generally is to demonstrate satisfactory performance under an overload above the design working value. This is usually judged by measurement of deflections under this load, which may be sustained for a specified period. The need may arise from doubts about the quality of construction or design, or where some damage has occurred, and the approach is particularly valuable where public confidence is involved. The load test was carried out on the repaired beams and the following procedure was followed: 

A system of steel pipes attached to steel plate was rigidly fixed at a Test location. A dial gauge mounted on a tripod, was fixed beneath the steel pipe. By this means any deflection of the structure upon loading, would immediately be transmitted and recorded on a dial gauge. Preliminary readings were taken before the test loads were applied.



The calculated test load was placed as a load consisted of bags of

Sand in layers,

on the roof (Fig 3.4). The load was placed in increments and sustained for period of 24 hours. At 24 hours final deflection readings were taken. 

the maximum deflection allowed by the is:456-2000 code 6 was Calculated Maximum deflection = l T2/2000xh.



The results showed that the allowable deflection equals 6.4mm and the Actual deflection equals 2.0mm.

The load test results showed that the deflections of the beams were within the allowable limits. The results indicate the effectiveness of the repair work, the integrity of those beams and they performed a well Performance under an overload above the design working value.

Fig 4. 2: Loading the Roof (Load Test)

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CHAPTER 5 PREVENTION 5.1 Prevention: Building materials expand on absorbing moisture and shrink on drying. These are generally reversible. Shrinkage in concrete or mortar depends on a number of factors 5.1.1

Cement Concrete:

Richer the mix greater is the drying shrinkage. 5.1.2

Water Content:

More water in mix induces greater shrinkage 5.1.3

Aggregates:

Large aggregates with good grading has less shrinkage for same workability as less water is used 5.1.4

Curing:

If proper curing starts as soon as initial set has taken place and continued for 7 to 10 days shrinkage is comparatively less 5.1.5

Excessive Fines:

More fines in aggregate requires more water for same workability and hence more shrinkage. 5.1.6

Temperature:

Concrete made in hot weather needs more water for same workability see Fig – 8 and hence results in more shrinkage.

5.2 Initial Expansion: An example of cracks of wall due to initial expansion of bricks is given in fig.

32

Fig 5. 1: Initial Expansion

5.3 Some Measures For Controlling Shrinkage: Shrinkage in plastering can be reduced by ensuring proper adhesion. The plastered should not be stronger than the back ground. Shrinkage cracks in masonry can be minimized by avoiding use of rich cement mortar and by delaying plastering till masonry has dried after proper curing and has undergone most of its initial shrinkage.

5.4 Thermal Movement: The cracking of a typical structure due to thermal movement is given in fig

33

Fig 5. 2: Thermal Movement In case of framed buildings due to thermal movement frames are distorted and cracks may appear as shown in fig

Fig 5. 3: Cracking In Cladding and Cross Walls of a Framed Structure

34

Fig 5. 4: Arching up and cracking of coping stones of a long garden wall

Fig 5. 5: Horizontal Crack at the Base of Masonry Parapet Supported On a Projecting RCC Slab

5.5 Prevention Of Thermal Cracks: To prevent thermal cracks expansion joints, control joints and joints in case of change of shape and direction of wing in a structure are to be provided

35

Fig 5. 6: Joint in floor slab

Fig 5. 7: Joint in roof slab

36

Fig 5. 8: H-Shaped Building & Rectangular shaped building Table 5. 1: General guide lines to provide movement joints TYPE OF STRUCTURE a

b

c

d

e

MOVEMENT OF JOINTS Provide 20 to 25 mm wide, joint at 10

RCC roof slab

to 20 M apart

Supports for RCC slabs 4 to 6M

Provide slip joint between slab and

length

bearing wall.

RCC framed structure, other load

Provide 25 to 40 mm wide expansion

and bearing structure

Joints at 30 to 45 M interval

Junction between old and new

Provide vertical slip joints.

structure

Expansion joint 5 to 8mm wide at 5to

Compound walls

8M interval and change of direction. Provide 20 to 25mm wide joints at 25m to 40m interval with control

f

Concrete pavement

joints at 5 to 8m. In cross direction control joints have to be provided at 3 to 5 m intervals.

g

Provide expansion joint 5 to 8mm

Chajja

wide at 4 to 6 M interval.

37

h

Provide expansion joints 5 to 8mm

RCC Railing

wide at 6 to 9m interval.

5.6 Elastic Deformation:

Fig 5. 9: Diagonal Cracks in Cross Walls of Multi-Stroried Load Bearing Structure

Fig 5. 10: Horizontal Cracks in a wall at supports due to excessive deflection of a slab of large span

5.7 Creep: Building items such as concrete and brick work when subjected to a sustained load not only undergo elastic strain but also develop gradual and slow time dependent deformation known as creep or plastic strain. The creep in brick work may stop after 4 months but the 38

same in concrete continue up to a year or so. The creep in concrete may be 2 to 3 times of the elastic deformation and hence has to be fully carefully considered.

5.8 General Measures For Avoidance Reduction Of Cracks Due To Elastic Strain, Creep And Shrinkage: 

Water cement ratio is to be controlled.



Reasonable pace of construction adopted.



Brick work over load bearing RCC members should be done after removal of shutting giving a time gap.



Brick walls between columns should be deferred as much as possible.



Plastering of areas having RCC and brick members should be done after sufficient time gap say one month or suitable groves provided in junction.



Shutting should be allowed stay for a larger period say 30 days or so for cantilevers which are bound to defect appreciably.

5.9 Movement Due To Chemical Reaction: 

Certain chemical reaction in building materials result is appreciable change in volume of resulting products and internal stresses are set up which may result in outward thrust and formation of cracks.



Soluble sulphate reacts with tricalcium aluminate in cement and hydraulic lime and form products which occupy larger volume and ends in developing cracks. An example of cracking of a floor due to coming in contact of the sub base made of brick khoa with heavy sulphate content and water can be seen in fig

39

Fig 5. 11: Cracking and upheaving of a tile floor due to sulphate action in base concrete

5.10 Some General Preventions: 

If sulphate content in soil is more that 0.2 % or in ground water more than 300 ppm use rich mix of concrete ant mortar has to be adopted.



Avoid bricks containing too much soluble sulphates (more than 5 %) and use rich mortar in such cases.



Use expansion and control joint at closure intervals

5.10.1 Corrosion Of Reinforcement: Corroded reinforcement expands and cracks the concrete cover. To avoid this phenomenon rich mix of concrete using proper quality of water and adequate cover should adopted. 5.10.2 Foundation Movement And Settlement Of Soil: Building on expansion clays are extremely crack prone. The soil movement in such clay is more appreciable up to a depth of 1.5 to 2M and this cause swelling and shrinkage and results in crack in the structure. The cracks due to settlement are usually diagonal in shape. Crack appearing due to swelling is vertical Fig

40

Fig 5. 12: Foundation movement and settlement of soil 5.10.3 Cracking Due To Vegetation: Large trees growing in the vicinity of buildings cause damage in all type of soil conditions. If the soil is shrinkable clay cracking is severe.

Fig 5. 13: Cracking due to vegetation

41

CHAPTER 6 EXPERIMENTAL WORK 6.1 General Observations: Actually we selected two sites for our project, one is located in university town Peshawar and the other one is in Kokarai swat which is masjid. Both the sites were observed for various possible reasons for instance excessive load, thermal expansion, sub-standard materials, improper designing, vibrational load due to earth quack and explosion in the nearby areas. 6.1.1

University Town Site Pictures:

42

Fig 6. 1: University Town Site Pictures

43

6.1.2

Swat (Kokarai) Site Pictures:

Fig 6. 2: Swat (Kokarai) Site Pictures

6.2 Test performed: Various test have been develop for curing cracks but we have selected two tests for convenience i.e. 

Ultrasonic test



Schmidth test

Both of these tests determine the width and depth of the crack. The figures illustrates the entire procedure of the test.

44

Fig 6. 3: Ultrasonic Test

Fig 6. 4: Ultrasonic Test

45

Fig 6. 5: Schmidt Test

Fig 6. 6: Schmidt Test

46

Fig 6. 7: Tell Tale Test

Fig 6. 8: Tell Tale Test

6.3 Conclusion from experiment: Upon comparing our research with the Dr. Qaiser Ali’s we have come to conclusions that crack in both our sites are due to shaking of ground either earth quack or explosions. University Town site is most probably due to substantial shake of the ground by explosions while swat site is because of earth quack back in 2005. Dr. Qaiser Ali also performed his research work on buildings demolish by earth quack in 2005. Following few figures illustrates his work, which has quite similar to our research work as well. 47

Fig 6. 9: Qaiser Ali's Research Work

Fig 6. 10: Qaiser Ali's Research Work

48

Fig 6. 11: Qaiser Ali's Research Work

6.4 Solution/Repair of the cracks The destruction caused be explosion and earth quack to both our sites were luckily not too much severe, so simply grouting or epoxy ejection proved very convenient for solution. Repair work is shown in the following figures.

6.5 Dry Packing It is the hand placement of low water motor into the slot or cracks. The motor should be placed in the layer about 3/8 inches thick. Each layer should be compact with the help of hammer or blunt stick. Curing is done for 14 days (ratio 1 cement 5/2 sand and water only to stuck the binding material together)

49

6.4.1 University Town Peshawar Repair Work:

Fig 6. 12: Repair Work (University Town)

Fig 6. 13: Univertity Town Repair Work 6.4.2 Swat (Kokarai) Repair Work:

50

Fig 6. 14: Repair Work (Swat - Kokarai)

Fig 6. 15: Swat (Kokarai) Repair Work

51

Conclusions Calamities like earth quack, floods have struck various part of Pakistan in the last decade or so which are also accompanied by bomb explosions etc. these calamities have either fully demolished the structures or left with cracks which are no longer serviceable and in appropriate for accommodation. Some of these buildings have taken substantial amount of money to build. These building cannot be demolished to the ground fully, but rather the proper ways have to be developed in order to bring them into service again. Repair work of the affected parts of the building will prove economical in comparison to totally grounding the building.

52

References [1]

Roger W. Johnson, “The Signifigance of Cracks in Low-Rise Buildings”, Ceng. Fistructe. Fice. Mrics, Chartered Civil and Structural Engineer and Surveyor.

[2]

Bonshor, R.B., Bonshor, L.L., BRE (1996), cracking in buildings, CRC Ltd.

[3]

BRE, (1991), Digest 361, why do buildings crack? CRC Ltd.

[4]

I. Struct.E. (2000), Subsidence in low-rise buildings, Instn.Struct.E. HQ. London.

[5]

Johnson, Roger W. (2001). Cracking in low-rise buildings – a methodology for diagnosing the cause.9th International Structural Faults and Repair Conference. (2001).Engineering Technics Press. Edinburgh.

[6]

B.B.Gamit, K.S.Krishnan, S.C.Nag, G.K.Sirohi, Shri.V.B.Sood, “Prevention & Repair of Cracks in Concrete Structures”, XEN(C) W.RLY, AEN (D) W.RLY, PT2, IRICEN.

[7]

Dr Liza O’Moore and Dr Peter Dux, “CRACK CONTROL – Are we getting it right?” The Department of Civil Engineering, the University of Queensland CIA Presentation, Sydney, 1 October 2003

[8]

TARSEM LAL, “CRACKS IN BUILDINGS CAUSES AND PREVENTION”, MASTER OF TECHNOLOGY IN STRUCTURAL ENGINEERING AT PTU REGIONAL

CENTRE

DAV

INSTITUTE

OF

ENGINEERING

&

TECHNOLOGY JALANDHAR, 2010 [9]

Aruna Munikrishna, Amr Hosny, Sami Rizkalla, and Paul Zia, “Behavior of Concrete Beams Reinforced with ASTM A1035 Grade 100 Stirrups under Shear”, ACI STRUCTURAL JOURNAL TECHNICAL PAPER, Title no. 108-S04, January-February 2011

[10]

Sasanka Choudhury and Dayal Parhi R, “Crack detection of a cantilever beam using kohonen network techniques”, Asst. Professor, Department of Mechanical Engineering, KMBB College of Engineering & Technology, Khurda 2Professor, Department of Mechanical Engineering, National Institute of Technology, Rourkela, Accepted June 2013

[11]

Dr. Qaiser Ali, “Earthquake Resistant Design of RC structures”, Earthquake Engineering Center NWFP UET Peshawar. 53

[12]

Dr. Qaiser Ali, “Affordable Earthquake Safe Houses”, Earthquake Engineering Center NWFP UET Peshawar.

[13]

Dr. Qaiser Ali, “Earthquake Safe house (concrete for roof)”, Earthquake Engineering Center NWFP UET Peshawar.

54